This invention relates to improvements in hemodialysis systems and, more particularly, relates to improvements in such systems with regard to apparatus for accurately proportioning and mixing two liquids. According to one general aspect of the invention, improved means are provided for controlling the proportionate relationship between the dialysate solution and the waste material drawn from the blood of the patient across the membrane of a dialysis cell in the classical hemodialysis procedure and, by controlling the flow rate of the solution, establishing a desired waste withdrawal rate. According to another general aspect of the invention, improved means are provided for the accurate proportioning and mixing of the liquids, namely, dialysate concentrate and dilution water, which constitute the component parts of the dialysate solution.
In the practice of hemodialysis, it is essential to control the rate at which waste materials are withdrawn from the blood since adverse patient reactions are experienced from too slow a withdrawal rate as well as from too fast a withdrawal rate. Depending upon a patient's size, weight, age, activity level and general physical condition, waste withdrawal rates of 5 milliliters to 15 milliliters per minute are sought by dialysis practitioners. The withdrawal rate is conventionally manipulated by adjustment of the flow rate of the dialysate solution through the artificial kidney and by adjustment of the relative liquid pressure levels on the two sides of the semi-permeable membrane in the artificial kidney, liquid on the dialysate side of the membrane being maintained at pressure levels that are negative with respect to pressure levels on the blood size of the membrane.
A typical dialysis system would, for example, be arranged for a dialysate flow rate of approximately 500 milliliters per minute through the artificial kidney for an average waste withdrawal rate of 10 milliliters per minute, or 2%, i.e., a ratio by volume of 50 parts of dialysate to 1 part of waste fluid.
The prior art practices for attempting to control the rate at which the waste materials are withdrawn are inadequate. One technique has involved reliance on empirically derived charts based on such factors as the weight of the patient in order to determine the appropriate dialysate flow rate. This technique is notoriously unreliable. Another technique has involved utilization of a filled system in which dialysate overflow has been presumed to be attributable to waste withdrawal through the membrane. Splashing, bubbling, gasification and general turbulence introduce inaccuracies into this technique. Another technique has involved positioning a load cell under the patient's bed and observing the weight loss of the patient on the presumption that the weight loss directly represents the blood wastes drawn across the dialysis cell membrane. This technique is subject to external influences; for example, suppose that a nurse gives the patient a glass of water or an additional blanket and omits to inform the artificial kidney operator. The operator would observe a reduced withdrawal rate and would increase the fluid flow possibly to an extent to result in a dangerous withdrawal rate. Yet another technique involves measuring variations in the electrical properties of the dialysate to either side of the cell. This, however, is not a direct measure of waste withdrawal and, consequently, is subject to inaccuracies.
Another disadvantage of the prior art is that single patient dialysis systems require bulky tanks for batch mixing of dialysate solution or require extensive electrically driven pumping systems to mix dialysate solutions with the line water fed to the artificial kidney unit. Commercially available dialysate concentrate is typically mixed with line water in a ratio of one part concentrate to 34 parts water in such installations.
Apparatus utilized in hemodialysis procedures for mixing liquids in accurate proportions is shown in Willock's U.S. Pat. No. 3,598,727 issued Aug, 10, 1971. In that apparatus, a double acting piston/cylinder unit has a pair of piston rods which extend from opposite ends of the cylinder in which the first liquid (usually water) is handled and each one extends into a corresponding one of two cylinders connected to a supply of a second fluid (a dialysate concentrate). The liquids are delivered through a spool valve to be mixed before being admitted to a dialysis cell.
While Willock's apparatus may be accurate, it of course requires multiple piston and cylinder units and complex valving to unite the liquids.